Alloys of germanium and method of making same



Patented Apr. 25, 1950 ALLOYS OF GERMANIUM AND METHOD OF MAKING SAMERandall M. Whaley, West Lafayette. ind.. as signor to Purdue ResearchFoundation, La Fayette, Ind., a corporation of Indiana Application March18, 1948, Serial No. 655,225

9 Claims. (Cl. 14813) My present invention relates to an improvement inalloys of germanium and method of making the same, which alloys haveutility as semiconductors in electrical devices sensitive to externallyapplied energy.

My present invention, aS will appear in greater detail hereinafter,concerns the combining of the gas, nitrogen, with germanium so that theresultant material is not an alloy in the common meaning of that word.However, for purposes of the present disclosure, it will be understoodthat the word alloy means to include a union of germanium and nitrogen,the combination of which exhibits electrical properties such as arefound in metals and semi-conductors.

As above indicated the alloys of my invention have use in electricaldevices with one of such devices known to me being a device sensitive toexternally applied energy, such as either light or heat, or combinationsof light and heat. The device is constructed like a point contact typerectifier in which a whisker or electrode has contact with the surfaceof a semi-conductor, the

latter in the present case being an alloy of my invention.

In the copending application of Karl Lark-Horovitz and myself, SerialNo. 604,744, filed July 13, 1945, there is disclosed various alloys ofwhich germanium comprises the principal constituent. As was disclosed inthe mentioned application, the germanium was obtained from theEagle-Picher Company of Joplin, Missouri, and had a purity of at least99%. The various elements which were to-be formed into the germaniumalloy semiconductor were then combined, mixed, or alloyed with thegermanium so as to form the type of unit herein to be described. Fromthese alloys rectifiers may be made which present high resistance tocurrent flow in one direction therethrough, and a low resistance tocurrent flow in the other direction. In that application, thesemiconductors disclosed are all of the type upon the surfaces of whichpoints may be found which exthat they are deliberately caused to havemany,

regions thereon exhibiting P-type electrical characteristics, i. e.,regions that produce rectifying contacts of polarity opposite to that ofthe N- type region, in addition to the already present large number ofN-type regions.

In the copending application of Seymour Benzer, Serial No. 651271, filedMarch 1, 1946, there is disclosed and claimed a device constructed inthe form of a point contact type rectifier and comprising asemi-conductor which may, if desired, be made of the alloys of myinvention. In the latter application it is disclosed for germanium-alloysemi-conductors having both P-type and N-type regions, that pointcontacts made in the transitions regions between the P-type and N-typeregions exhibit a change in current-vo tage characterist cs whensubjected to externally applied energy in the form of light or heat, orboth. The dev ces of that application exhibit the sensitivecharacteristics when the potential on the semi-conductor is negative andthe potent al upon whisker positive. It has been discovered, for suchpolarity of the semi-conductor and electrode, that selected po nts ofcontact in the transition region of the semi-conductor exhibit thefollowing characteristics when exposed to light or at a temperature ofpredetermined values. As the current is progressively increased. theresistance to current flow in the low resistance direction is positiveup to a certain point hereinafter referred to as the trigger peak, afterwhich the differential resistance becomes negative for a certain rangeof currents and then becomes positive again. Points may be found in thetransition region of the semi-conductor where the trigger peak occurs atsmall currents of the order of 0.1 to 5 m lliamperes and at voltages of0.1 to 25 volts depending upon the selected point of contact on thesurface of the semi-conductor.

This range of values may be observed with the atoms;

about volts in the dark at ambient room temperature, and with thattemperature being maintained substantially constant, it has been foundthat the trigger peak will be reduced to about 3 volts when white Mazdalight of an intensity of about 0.5 lumen/cm. is projected on theselected point of contact. If stronger light of about 2 lumen/cm. whitelight is projected on the contact the differential negative resistanceis entirely removed. The same eflects may be observed with the devicemaintained in the dark upon progressively increasing the temperature ofthe rectifying contact. For the above selected illustrative point ofcontact the trigger peak at 5 volts occurs at a temperature of about 25C. Upon increasing the temperature to about 50. C. the trigger peaksoccur at about 3 volts. Further increase of the temperature to the orderof 100 C. causes the complete removal of the negative resistancecharacteristic. This behavior of the device may be utilized to make arelay type of photosensitive or thermosensitive cell. device may, forexample, be biased to closely approach the trigger peak of a selectedpoint of contact. Now when light or heat of a predetermined amount isapplied to the selected point of contact the trigger peak disappears sothat a trigger effect occurs with the current jumping to a much highervalue. A jump in current by as much as a factor of 500 has been obtainedwith selected points of certain of the germanium crystals made of thealloys of my invention. Upon removal of the light or heat the electricalcharacteristics return to their original forms and values.

However, if a D. C. potential is applied to the semi-conductor andelectrode-it must be removed for an instant to effect restoration of theoriginal currentvoltage characteristics. When alternating current isused, however, the restoration is automatically achieved each cycle. Theuse of alternating current is possible since the current-voltagecharacteri tic when the semi-conductor is positive and the contactingwhisker or electrode is negative remains at a relatively high resistanceeven under changes of light and heat on the selected point of contactproducing the aforesaid typical chan es in the current-voltagecharacteristic on current flow in the other direction. Thus t e currentproduced will automaticallv be rectified when the device is actuated byexternally applied energy either in the form of light or heat.

It is an ob ect of my invention to provide germanium-nitrogen alloyshaving P-type and N-tvne re ions with transitions regions therebetweenwithin which point contacts can be made which are sensitive toexternally applied energy.

A further obiect of my invention is to provide a method of makinggermanium-nitrogen alloys having a large number of P-type and N-typeregions with transition regions therebetween wherein point contacts canbe made, the electrical characteristics of which are sensitive toexternally applied energy.

A further object of my invention is to provide a method of making alloysof germanium-nitrogen having a large number of P-type and N-typeregions.

The above and other objects and advantages of my invention will appearfrom the detail description.

Now, in order to acquaint those skilled in the art with the manner ofpracticing the method of my invention and producing the alloys thereof,

The.

I shall describe a preferred embodiment of the method and resultingalloys of my invention.

In order to provide a clear understanding of my invention, I shalldescribe the use alloys have in constructing a device sensitive toexternally applied energy. shown in the accompanying drawings in which-Figure 1 is a diagrammatic view of a device sensitive to externallyapplied energy in which the semi-conductor element thereof is made of analloy of my present invention;

Figure 2 is a graphic illustration of the progressive changes in theelectrical characteristics for point contacts made at different pointsin a transition region between P-type and N- type regions on the surfaceof a semi-conductor made of an alloy of my invention;

Figure '3 is a graph illustrating the'effect of light or heat upon theelectrical characteristics of the device of Figure 1 having the semi-comductor element thereof made of an alloy of my invention;

Figure 4 is a graph similar to Figure 3 with the current scale reducedand illustrating the trigger effect which may be obtained with thedevice of Figure 1;

Figure 5 is a typical simple circuit diagram in which the device ofFigure 1 may be embodied; and

Figure 6 is a diagrammatic illustration of what is believed to be thephysical structure of the semi-conductor of Figure 1 showing the whiskeror electrode having contact with a barrier between the' P and N-typeregions in the semiconductor element accounting for the external energysensitive character of the device.

In a preferred embodiment of the method of my invention I propose toimpregnate germanium with nitrogen at pressures ranging from about 2 mm.to 760 mm. mercury at temperatures in the neighborhood of 1000 C. to1100 C. for about 5 to 15 ,minutes. This alloy and the method ofpreparing the same is disclosed in the above referred to copendingapplication of Karl Lark- Horovitz and myself, to which reference may behad. As stated in the said copending application, Serial No. 604,744,the impregnation is carried out with the germanium being in the moltenstate. The melting point of germanium is known to be 958.5" C. Asdisclosed in that application. points of contact may be readily found onthe surface of such an alloy which exhibit Ntype rectification. Also asdisclosed in that application, it is preferable in practice to grind aflat surface on the alloy and then etch the surface to facilitatefinding of good points of contact. One suitable abrasive for grinding asurface of the alloy is 600 mesh alumina (A1203). Various etchingsolutions and techniques may be used and it has been found that an etchconsisting of four parts of hydrofluoric acid (48% reagent), four partsdistilled water, two parts concentrated nitric acid, and. 200 milligramsCu(NO3)2 to each 10 cc. of solution is satisfactory. This solution willproduce a satisfactory etching of the ground alloy surface in about oneto two minutes. The allow is then, in accordance with the novel step ofmy present invention, heat treated in vacuum at about 650 C. to 750 C.for about one-half hour to one hour. This latter step is additional tothe steps in the method of preparation of the germanium-nitrogen alloysset forth in the above application of Lark-Horovitz and myself. I havefound that this heat treatment is particularly effective in producingmany P-type regions on the surface of the alloy as distinguished fromthe appearance of mostly N-type regions thereon prior to the latterstep. In the transition regions between the P and N-type regions thuscreated, a substantial number of sensitive points may be found,facilitating the construction therefrom of the devices sensitive toexternally applied energy to be hereinafter referred to and forming thesubject matter of the above referred to application of Seymour Benzer.

Referring now to the drawings, I have shown in Figure 1 an electricaldevice 2 comprising a semi-conductor 3 made of a germanium-nitrogenalloy in the manner above described. The semiconductor 3 is preferablysoldered to a conductor plate i of any metal or alloy having goodelectrical conductivity characteristics. An electrode or whisker t isformed with a pointed end which makes point contact with the surface ofthe semiconductor 3. The electrode or whisker 5 may be made of metal oran alloy having good electrical conductivity and preferably of goodmechanical strength. Several metals that have been found satisfactoryfor use in making the electrode or whisker are tungsten, platinum,copper, iron, gold. silver, manganese, tantalum, nickel, zinc,molybdenum, zirconium, lead, and platinumiridium alloys. The electrode 5preferably is provided with a sharp pointed end havinga tip diameter ofthe order of 0.0001 to 0.001 inch. These electrodes may be readily madeby forming suitable points on wires of about 0.005 inch in diameter. Alead 6 is suitably connected to the electrode t and a lead i hasconnection with the conductor plate t upon which the semi-conductor ismounted.

As above mentioned, the points on the surface of the semi-conductorhaving properties sensitive to both light and heat may most often befound in the transition regions separating N and P-type areas orregions. Figure 2 of the drawing illustrates typical curves of thechanges in the current-voltage characteristics of rectifying contactsmade at points progressing from a P- type region to an N-type region.For purposes of constructing an electrical device sensitive toexternally applied energy, the characteristic indicated by curvedesignated by the reference numeral it has been found to be the mostsuitable.

The effect of light with the temperature being held constant at about 25C. is illustrated in Figures 3 and i of the drawing. In these graphs thecurrent-voltage characteristic for a selected point of contact betweenthe electrode t and the semi-conductor it, while being maintained in thedark, is shown by the curve A. It will be observed that in Figure 4 thecurrent ordinate is considerably reduced with respect to the currentordinate of Figure 3. Curve B illustrates the current-voltagecharacteristic when white light of about 0.5 lumen/cm. is projected onthe point of contact between the whisker and the selected point on thesemi-conductor 0. Curve C illustrates the current-voltage characteristicwhen white light of about 2 lumens/cm. is projected on the selectedpoint of contact of the whisker with the semi-conductor from which itwill be seen that the trigger peak is completely removed. It will beobserved from the curves A and B of Figure 3 and curve A of Figure 4that as the current through the device is progressively raised, theresistance is positive up to a certain point, herein designated as thetrigger peak after which the differential resistance becomes negativefor a certain range of values and then illustrates the trigger effectcharacteristic. Un-

der a D. C. bias of about 2 volts there is for most practical purposesno passage of current of any appreciable amount. However, if white lightof about 0.7 lumen/cm. is projected on the selected point of contact,the current immediately jumps to a value of about 20 milliamperes whichis sufllcient to effect operation of a relay or other electricalapparatus with which the device may be connected in circuit. The currentjump is shown in Figure 4 with the point P1 illustrating the current ata D. C. bias of 2 volts across the selected point of contact of thewhisker 5 with the semi-' conductor 3 and with the device in the dark.Now when light of about 0.7 lumen/cm. is projected on the selected pointof contact, the current immediately jumps to the point F2 on curve Aproviding for current flow of the order of 20 milliamperes. The aboveset of values, it will be understood, are only representative of one setof conditions obtainable on a selected point of contact between theelectrode and semi-conductor, and it will be understood that otherpoints of contact having characteristics lying within the above rangesmay be found by probing of the transition region of the alloys of myinvention. Oi course, different D. C. bias voltage may be used withinthe range above set out for selected points on the semi-conductors.

As a general observation it may be stated that for a contact ofrelatively good sensitivity to light, the voltage of the trigger peakwill fall to one-half of its dark value when it is illuminated withabout one lumen per square inch of white light. This corresponds to theintensity of illumination produced by a watt Mazda lamp at a distance ofone foot. Only light which falls within the immediate neighborhood ofthe point contact is effective. Therefore, a much weaker source ofillumination may be used provided the light is condensed to a smallspot. As: is usual in photosensitive devices, the sensitivity varieswith the wave length of the light used. The device herein describedshows maximum sensitivity in the neighborhood of 1.3 microns, whichfalls in the near infra-red region of wave length. It is of practicalinterest to know that the spectral intensity distribution of a Mazdalamp has its maximum in the same region of wave length.

The eflect of temperature with the illumination held constant is alsoillustrated by curves A, B and C of Figures 3 and 4. With the devicemaintained in the dark at a temperature of about 25 (2., points may befound on the alloys of my invention which will give the currentvoltagecharacteristics shown by curve A of the values previously stated andshown in these figures. As the temperature is raised to the order of 50C. for the selected point of contact, the current-voltagecharacteristics will be that of curve B. As the temperature is raisedstill further to that of the order of 100 C., the current-voltagecharacteristics will be that of curve C.

It is believed that from the above it will be clear that the device mayalso be caused to be acoacaa actuated by combinations of applied lightand heat.

A typical circuit embodying the device of Figure 1 is shown in Figure 5of the drawings. As shown in this figure, the circuit comprises a sourceof alternating current or direct current 19 from which a lead 20 extendsto a germaniumnitrogen alloy semi-conductor 2| of the type abovedescribed having the properties previously referred to. An electrode 22makes a rectifying contact with the semi-conductor 2|. A lead 23 extendsfrom the electrode 22 to a current responsive device 24 which in turn isconnected by lead 25 to the source of current IS. A source of light 26which may be concentrated by a lens 21 is directed upon the selectedpoint of contact between the electrode 22 and the semi-conductor 2|. Theintensity of the light may be varied in any suitable manner to actuatethe device, which in turn will alter the current flow through thecurrent responsive device 24'. The latter device may, for example, be acurrent indicator or a relay to be actuated by the current flow, forexample, of the value P: of Figure 4. It is understood that a heatsource or means 28 may be substituted for the light source 26, as aboveexplained, to provide a thermosensitive device within the ranges aboveset forth to control the current responsive device 24 or, if desired,various combinations of both light and heat from sources 26 and 28 maybe utilized to effect actuation of the device.

In Figure 6 I have shown diagrammatically an arrangement of an electrode30 having contact with a selected point on the semi-conductor 3|. It isthought that the germanium-nitrogen alloys of my invention havebarriers, such as designated by numeral 32, extending between andseparating P-type and N-type regions of the semi-conduc tor. It isbelieved, although at present not definitely ascertained, that theenergy sensitive characteristics occur when the electrode has contactwith the semi-conductor at the point where the barrier meets the surfaceof the semi-conductor. At any event, points may be found in thetransition region between N-type and P-type regions in the alloys of myinvention which possess the electrical effects above described and inthe ranges stated.

Further, little is known of the effect impurities such as nitrogen havein the germanium. It is thought that the nitrogen is ionized andaccounts in some manner for the conductivity of the alloy. However, itis quite feasible that the effect could be due to a change in latticestructure of the germanium due to the presence of the nitrogen impuritytogether with further changes due to the heat treatment which accountfor the results obtained.

I claim:

1. The method of making a semi-conductor having transition regionsbetween N-type and P-type regions of the character described comprisingalloying nitrogen with germanium having a purity of at least 99% atpressures ranging from 2 mm. to 760 mm. Hg at a temperature of 1000 C.to 1100 C. for 5 to about 15 minutes to form an alloy having anabundance of said N-type regions, and then heating the alloy thus formedin vacuum at a temperature of 650 C. to 750 C. for about one-half hourto one hour to produce a plurality of said P-type regions in said allow.

2. The method of making a semi-conductor having transition regionsbetween N-type and P-type regions of the character described comprisingalloying nitrogen with germanium having a purity of at least 99% atpressure ranging from 2 mm. to 760 mm. Hg at a temperature of 1000 C. to1100 C. for 5 to about 15 minutes to form an alloy having an abundanceof said N-type regions, etching a surface on the alloy thus formed, andthen heating the alloy thus formed in vacuum at a temperature of 650 C.to 750 C. for about one-half hour to one hour to produce a plurality ofsaid P-type regions in said alloy.

3. An alloy consisting of germanium having a purity of at least 99% andnitrogen characterized by nitrogen having been impregnated in thegermanium at pressures ranging from 2 mm. to 760 mm. at a temperature of1000 C. to 1100 C. for 5 to about 15 minutes and the alloy thus formedhaving then been subjected to heat treatment in vacuum at a temperatureof 650 C. to 750 C. for about one-half hour to one hour to providetransition regions the resistance of which is sensitive to externallyapplied energy, such as heat or light, between P-type and N-type regionsthereof of the character described.

4. An alloy consisting of germanium having a purity of at least 99% andnitrogen characterized by nitrogen having been impregnated in thegermanium at pressures ranging from 2 mm. to 760 mm. at a temperature of1000 C. to 1100 C. for 5 to about 15 minutes, with a surface of thealloy thus formed having been etched, and said alloy then having beensubjected to heat treat-' ment in vacuum at a temperature of the orderof 650 C. to 750 C. for about one-half hour to one hour.

5. The method of making a semi-conductor having transition regionsbetween N-type and P-type regions of the character described comprisingalloying nitrogen with germanium having a purity of at least 99% atpressures ranging from 2 mm. to 760 mm. Hg at a temperature of 1000 C.to 1100 C. for 5 to about 15 minutes to form an alloy having said N-typeregions, etching a surface on the alloy thus formed with an etchcomprising four parts hydrofluoric acid (48% reagent), four partsdistilled water, two parts concentrated nitric acid and 200 milligramsCll(NO3)2 to each 10 cc. solution for about one to two minutes, and thenheating the alloy thus formed in vacuum at a temperature of 650 C. to750 C. for about one-half hour to one hour to produce said P-typeregions in said alloy.

6. The method of making a semi-conductor having points thereon betweenN-type and P-type regions of the character described, the electricalresistance characteristics of which points change in response toexternally applied energy, said method comprising alloying at pressuresranging from 2 mm. to 760 mm. Hg nitrogen with germanium having a purityof at least 99% by impregnation of said germanium, in the molten state,with said nitrogen to form an alloy having said N-type regions, grindinga surface of the alloy thus formed, etching said surface of said alloy,and then heat-treating the alloy thus formed in vacuum at temperaturesin the range between 650 C. and 750 C. to produce therein said P-typeregions.

7. The method of making a semi-conductor upon which point contacts maybe made, the resistance of which is varied by varying the intensity ofexternally applied energy comprising, etching a surface of an alloyconsisting of germanium having a purity of at least 99% and nitrogen,and then heat-treating the alloy in vacuum at a temperature in the rangebetween tomes 650 C. and 750 C. for one-halt hour to one hour.

t. The method of making a semi-conductor upon which point contacts maybe made, the resistance of which is varied by varying the intensity ofexternally applied energy comprising,-

9. In a method of making a semi-conductor having transition regionsbetween N-type and P-type regions of the character described, from analloy consisting essentially of nitrogen and of germanium having apurity of at least 99%, said alloy having only N-type regions therein,the step of heat treating said alloy in vacuum at a temperature of 650to 150 C. for about one-halt hour to onehour, whereby said P-typeregions are formed.

RANDALL M.

REFERENCES CITED The following references are of nieof this vpotent:

record .in the

1. THE METHOD OF MAKING A SEMI-CONDUCTOR HAVING TRANSITION REGIONSBETWEEN N-TYPE AND P-TYPE REGIONS OF THE CHARACTER DESCRIBED COMPRISINGALLOYING NITROGEN WITH GERMANIUM HAVING A PURITY OF AT LEAST 99% ATPRESSURES RANGING FROM 2 MM. TO 760 MM. HG AT A TEMPERATURE OF 1000*C.TO 1100*C. FOR 5 TO ABOUT 15 MINUTES TO FORM AN ALLOY HAVING ANABUNDANCE OF SAID N-TYPE REGIONS, AND THEN HEATING THE ALLOY THUS FORMEDIN VACUUM AT A TEMPERATURE OF 650*C. TO 750*C. FOR ABOUT ONE-HALF HOURTO ONE HOUR TO PRODUCE A PLURALITY OF SAID P-TYPE REGIONS IN SAID ALLOW.